The invention relates to a lithium-water battery for providing reliable power for long durations of time in an aqueous environment, and particularly in the ocean, which serves as a reactant water source. It is particularly useful in low discharge rate applications of about 100 mA/cm.sup.2 (anode area) or less.
Magnesium-water batteries are presently the preferred autonomous undersea power sources for use in low rate, long duration applications. A magnesium-water electrochemical system has a high theoretical specific energy of about 5200 watt-hours/kilogram of magnesium. Also, reactant water need not be prepackaged in the battery because water is freely available from the aqueous environment. Thus magnesium-water batteries have a high energy/weight ratio.
Certain magnesium-water batteries have proven to be unreliable in deep ocean environments. It was found that the magnesium-water electrochemical reaction requires the presence of dissolved oxygen to depolarize the cathode. Thus the performance of these batteries begins to deteriorate where the dissolved oxygen content of the water is less than about 2.0 ml O.sub.2 /liter. Their performance drastically declines where the dissolved oxygen content of the water is less than about 0.5 ml O.sub.2 /liter. Silver chloride may be employed in magnesium batteries as the cathodic reactant, but this requires that the silver chloride be prepackaged and carried by the battery. Thus a magnesium-silver chloride battery incurs a substantial weight penalty which penalizes its specific energy (energy/battery weight ratio). However, a battery containing an anode of magnesium or a similar non-reactive metal and an oxidizer such as silver chloride or the like is stable and relatively safe to handle.
The unreliability of magnesium-water batteries in deep ocean low power environments has created a specific need for an all-ocean battery based on another technology which is capable of providing several watts at more than about 0.75 volts for a minimum duration of one year. Such a battery must reliably provide stable power at ocean depths of up to 6000 meters (20,000 feet) and at temperatures down to 0.degree. C., where the oxygen content of the ocean may be less than about 0.5 ml O.sub.2 /liter. Importantly, a long duration, low power battery system must be lightweight so that it may be readily handled. This limitation on weight requires that a low power battery system have a practical specific energy of at least about 770 watt-hour per kilogram of battery weight.
It has been proposed to employ a low rate battery based on a lithium-thionyl chloride electrochemical system to replace the magnesium-water batteries. However the best low rate lithium-thionyl chloride systems only provide a system specific energy about 450 watt-hours/kilogram. Thus these systems fall short of the 770 Wh/kg limitation. In addition, lithium and thionyl chloride are highly reactive. Thus batteries based on these systems must be carefully stored and handled.
A lithium-water battery system would be a logical replacement for the magnesium-water system if water were not so corrosive toward lithium. The highly exothermic (-53.3 kcal/gm-mole lithium) corrosion reaction proceeds in accordance with the following equation: EQU Anode: Li+H.sub.2 O.fwdarw.LiOH+1/2H.sub.2.
The open circuit corrosion current density of this corrosion reaction in seawater at 0.degree. C. and ambient pressure is about 19,500 mA/cm.sup.2, which will approximately double for each ten degree centigrade temperature rise. Further, corrosion losses tend to accelerate with time due to the resultant nonuniform lithium surface morphology. This generally results in premature loss of performance and premature end of life before the lithium is fully utilized. The electrochemical reaction, which generates useful energy, proceeds in accordance with the following equations: EQU Anode: Li.fwdarw.Li.sup.+ +electron EQU Cathode: H.sub.2 O+electron.fwdarw.OH.sup.- +1/2H.sub.2 EQU Overall: Li+H.sub.2 O.fwdarw.LiOH+1/2H.sub.2
A present high power lithium-water battery system, which has been demonstrated to be useful in short duration high power applications, discharges at a current density of 800-1000 mA/cm.sup.2 of lithium surface. This system achieves lithium utilization rates of over 80%. The lithium hydroxide may then precipitate as a monohydrate crystal if the lithium hydroxide solution concentration is allowed to reach saturation.
The present short duration, high power, lithium-water battery system controls corrosion of the lithium anode by circulating a 4-5 molar LiOH electrolyte solution to maintain a suitably porous oxide film on the surface of the anode. At concentrations less than about 4M LiOH, the corrosion rate becomes excessive. At concentrations above 5M LiOH, which is near the saturation point, the performance of the battery system is unacceptable. In addition, the LiOH electrolyte solution can contain about 5%-20% by volume methanol. It is theorized that the thermodynamic activity of water is reduced via hydrogen bonding with methanol to a point where the water will not aggressively attack the lithium anode. The composition of the electrolyte solution is maintained by a mechanical pump which introduces make-up water from the aqueous environment into the battery system to replace the reacted water. The mechanical pump also circulates the electrolyte solution through the system and bleeds a small amount of electrolyte solution from the system back to the aqueous environment in order to maintain the lithium hydroxide concentration below the saturation point of the electrolyte solution. The anodic reactions are so sensitive to the flow rate of the electrolyte solution, that the voltage may be controlled by the pumping rate.
The present short term, high power, lithium-water battery system has been demonstrated to be useful for durations of less than one hour. However, this battery system is probably not practical for durations of more than a few days because of the questionable ability of a mechanical pump to reliably operate for long durations without any maintenance. This is a particular concern in an all-ocean battery system which must reliably operate at ocean depths of down to about 6000 meters (20,000 feet) and at temperatures of down to about 0.degree. C. However, if a mechanical pump is not employed, then natural convection must be employed to circulate the electrolyte solution in order to maintain a suitably porous dynamic film on the lithium anode. Also there must be a means for introducing make-up water into the battery system to replace the water which reacts with the lithium and the lithium hydroxide.
A practical long duration, low power, lithium-water battery system will have a low electrochemical current density of less than about 100 mA/cm.sup.2 of anode surface. Thus a suitably porous dynamic film must be carefully maintained on the anode to control corrosion without passivating the anode. Further a lightweight battery system must have a lithium utilization rate of at least about 25% in order to realize an overall specific energy of 770 Wh/kg or more based on the total system weight.